Means for testing the durability of material which is exposed to repeatedly-applied stresses.



G. KAPP. MEANS FOR TESTING THE DURABILITY 0F MATERIAL WHICH IS EXPOSEDT0 REPEATEDTIY APPLIED STRESSES.

APPLIOATIOH FILED OCT. 14. 1911.

HlllllHHHllllll A i I l i z I I I I I I: I I II 6 l :1 I II I I: l I f 1WITNessEs: a a N zMM e UNITED STATES PATENT OFFICE.

GISBER'I KAPP, OE BIRMINGHAM, ENGLAND.

MEANS FOR TESTING THE DUBABILITY 0F MATERIAL WHICHIS EXPOSED TOREPEATEDLY-APPLIED STRESSES.

Specification of Letters Patent.

Patented May 27, 1913.

To all whom it may concern:

Be it known that I, GIsBnR'r 'KAPP, subject of the K' of Great Britain,residing at Pen-Y-Coe dfi 'ritchatts Road, Edgbaston, Birmingham,England, have invented new and useful Im rovements in Means forTestingthe Durability ofMaterial Which is Exposed to Repeatedly-AppliedStresses, of which the following is a specification.

This invention relates to improvements in means for testing thedurabilit of material which is exposed to repeated y applied stresses.In the contrivances which have hitherto been employed for this purpose,the conditions have been such as to preclude a very high frequency ofrepetition of stress, causing the process of ascertaining the measure ofthe strength of the material to withstand unlimited repetition of a likestress to be very prolonged and tedious.

By the present invention, in which the stress is due to the applicationof a magnetically produced force derived by an alternating electriccurrent I am able to raise the frequency of the application of thestress to such a degree as to render the length of time occupied in theexpleriment only a small fraction of that hit erto necessary, while, atthe same time, the stresses which, in the systems of operation hithertoemployed, are due to change of momentum of the moving parts. of theapparatus, are almost entirely eliminated. Moreover, by varyin thefrequency and the E. M. F. of the a ternatin current I am able tovarythe intensity 0 the stress applied and obtain a recise indicationthereof by a voltmeter w ich can be calibrated to indicate directly thestress or load ap lied.

An a ternating current sent through the winding of an electro magnetproduces a maximum induction alternated by zero induction twice in eachperiod. The armature of 'such a magnet is therefore attracted with amaximum force twice in each com lete period and at intermediate timesthere is no attractive force. If the armature is so connected to a testpiece as to cause the latter to be strained by the said attractiveforce, the test piece will be subjected to a stress varying between zeroand a maximum twice in each eriod. With a current of 5Q frequency orexample, the test plece will be subjected to 100 stresses a second, orover a million in three hours. I mention the frequency of 50 only by wayof example; any other frequency may be used, provided it is not so highas to produce resonance with the natural frequency of vibration of thetest piece and the mass of the armature ata footlong and the minimummass of armature requisite, the natural period of vibration is soextremely short that even a higher frequency than 50 could be employedwitht-ached thereto. With test pieces less than out lncurrlng anyexaggeration of the stress by resonance.

On the accompanying drawing are shown, as representative examples,contrivances adapted to efiect' the erformance of the above describedinvention in three specific ways, the nature of the straining actionsand their variations being those which most commonly occur inconstructions subject to repeated loads. The method of testing,particularly illustrated and described, will be applicable, withsuitable obvious modifications of the construction of the apparatus, totesting material which is subjected to other kinds of straining actionsand other degrees of variation of their intensit ample, shearing andtorsion an compression alternated with tension. Moreover the precisionof the instant at which the maximum stress occurs, under the abovedescribed sys tem of operation, and the absence of all lateralconstraint of the armature to which longitudinal force is applied,renders the method particularly available for throwing light on theefiect of the application of compound stresses to materia for a specimencould in a suitably constructed machine easily be simultaneouslysubjected to repeated stresses in directions at right angles to eachother.

as for exwhich the number of repetitions to efl'ect destruction isdirectly ascertained or, alternatively, a plurality of like test piecesmay be sub ected to a repeated stress a number of times which differ ineach specimen and the efiect of the repeated stresscdetermined byobserving difierences in the behavior of the specimens when they aresubsequently tested in the ordinary way,

Injhe drawing :Figures 1 and 2 are two views, taken at right angles toone another, of a machine for subjecting a test piece to a rapidsuccession of tensile stresses alternated with zero stress. 8 shows amodifled-construction whereby a test piece can be subjected to arepeated bending stress in one direction alternated with zero stress,

and Fig. 4 shows a further modification whereby a bending stress in onedirection is alternated by a bending stress in the contrary direction. r

Referring to Figs. 1 and 2, a is the yoke and b b are the polar limbs ofa magnet built up of laminated iron plates. The polar limbs b b aresurrounded by the exciting coils c c. The electro-magnet a b b issecured to a cross-head d by means of plates 0 e which plates serve alsoas guides for the armature f of the electr0-magnet which armature isalso built up of laminated iron plates. The test piece 9 to which thetensile stress is to be applied 1s secured at one end to the cross-headd and at the other end it is secured to the armature f by a palr ofcheek-plates h 72. having interposed distancepieces k k bolted togetherand to the cheekplates by the bolts k 722*.

In orderto subject a test-piece) to repeated bending in one direction1t- IS inserted in the machine transversely as shown "in Fig. 3 andsupported on two bearingblocks k In which are secured between thebefore-mentioned plates e e by cross-bolts m. The load is applied to thecenter of the length of the test-piece by a block n which is insertedbetween and bolted to the beforementioned cheek-plates h h by the bolts72. h. The electro-magnet a b b, exciting coils c cand armature f aredenoted as in Figs. 1 and 2. Fig. 4 shows. how the testpiece a issubjected to bending in alternatively opposite directions. For thispurpose the'electro-magnet, exciting coils and armature are provided indu licate as indicated by a b b, 0 a, f and 2 b 0' 0 F. Thebearing-blocks to support the test-piece at each of its two ends arealso duplicated as indicated by k, k 70 So also is the loading block asindicated by n at, both blocks being secured between cheek plates whichunite the two armatures.

In either case the test-piece is so attached to the armature that thelatter can move only through the very small distance correeponding tothe strain produced by the tensional or other stress imposed on thetestand since also the E. M. F. required to overcome ohmic resistance issmall and in quadrature with the E. M. .F. of self-induction, it followsthat the E. M. F. of self-induction is very nearly equal to the terminal.E. M. F. and that therefore the square of the E. M. F. supplied to thewinding of the magnet is proportional to the stress produced. Theapparatus may therefore be calibrated for a particular frequency andwave form of the impressed E. M. F. by observing corresponding values ofE. M. F. and the force required, (measured by dead weight or sprinbalance), to tear off the armature from t e poles, a sheet of fiber ofequal thickness to the normal interpolar airspace having been reviouslyinserted. If the E. M. F. wave as approximately sine form the crestvalue of the attractive force is twice the measured, or average, value.If the wave form is irregular, the crest value of the attractive forcecan be determined a from an oscillogram in the manner well known toelectrical engineers.

A sli ht difference in the thickness of the interpo ar space will no.aifect the relation between E. M. F. and. attractive force, it onlyaffects the strength of the exciting current. The indication of thevoltmeter may therefore be used as an indication of the stress impressedon the sample. At a given frequency the square of the voltmeter readinmultiplied b .a constant previously determined by the ca ibration hereex lained gives the stress, and by suitably gra uating thescale of thevoltmeter the stress impressed on the test-piece may be read ofidireqtvly from the face of this instrument.

en it is desired to subject a test-piece to stresses which arealternately of opposite characters as indicated in Fig. 4, the twoarmatures are rigidly connected and exert a force in one direction orthe other accord ing to which of the magnets is excitedf B y excitingthe magnets with currents having a phase diiference of a quarter periodthe magnet on one side will exert its maximum of attractive force at thetime when the other is inert and vice-versa. The two armatures rigidlyconnected to each other are thus alternate y forced in oppositedirections and the testiece is acted on by stresses alternately oopposite character.

The currents re uired to energize the apparatus may be ta en from anyconvenient source of supply, provided that wave form, E. M. F. andfrequency retain the values which they had when the apparatus wascalibrated. Where this condit1on is not fulfilled I employ aspecialtwohase alternator driven at approximate y constant speed by anyconvenient meansggand Iregulate the stress by adjusting the excitationof the alternator. This may be done by a series or shunt rheostat in amanner well known to electrical engineers. In this case a small chan eof speed does not result in a change 0 induction because the E. M. F.produced by the alternator and the selfinduced E. M. F. of the magnetsfollow the same law of proportionality with the speed; hence the stressimpressed upon the test piece is dependent only on the excitation of thealternator and is almost independent of its 5 eed. I attach a revolutioncounter to the s aft of the alternator and by this means I determine thetotal number of stresses which have been impressed on the test-piece inany given time that the apparatus has been at work.

The double magnet apparatus may also be used for the production ofstresses in one direction only; if this is desired no other alterationis required than the switching off of one of the magnet circuits.

1 claim:

1. A contrivance whereby the material of a test-piece may be subjectedto a rapid succession of repeated stresses comprising an.electro-magnet, the armature thereof,

means for energizing the electro-magnet by an alternating electriccurrent and means for securing a test-piece to both the armature and theelectro-magnet.

2. A contrivance whereby the material of a test-piece may be subjectedto a rapid succession of repeated stresses comprising an electro-magnet,the armature thereof, means for energizing the electro-mag'net by analternating electric current, and means for securing a test-piece toboth the armature and the electro-magnet so that the sti ain of thetest-piece is equal to the displacement of the armature relative 'to theelectro-magnet.

3. A machine whereby the material of a test-piece may be'subjected to arapid succession of repeated stresses by a load applied in alternatelyopposite directions, comprising two electro-magnets, the armaturesthereof, means for securing one portion of the test-piece to theelectro-magnets and another portion to the armatures, and means forenergizing the electro-magnets by alternatin'g electric currents.

In testimony whereof I have signed my name to this specification in thepresence of two subscribing witnesses.

GISBERT KAPP.

Witnesses:

EDGAR J. Krrrs, THos. F. WALL.

